Coating a fluid onto a web of material is well known. Such coating can often be conveniently done using a coating die having a cavity communicating with an applicator slot. Liquid under pressure is introduced into the cavity, and is then extruded out of the applicator slot onto a desired substrate.
Depending on the exact result desired, variations on this theme are numerous, with various coating aids being known. In particular, it is known that under certain conditions, particularly when the speed of the web past the coating die is very rapid, the material dispensed from the applicator slot may neck inwards erratically. One parameter that may be predictive of whether this necking will occur is the so-called “capillary number” characteristic of the coating process.
The capillary number is a dimensionless parameter defined as:
  Ca  =            μ      ⁢                          ⁢      V        σ  where Ca is the capillary number, μ is the viscosity of the material dispensed or coated at the characteristic shear rate of the coating process, V is the speed of the moving web or other substrate, and μ is the surface tension of the material. At higher capillary numbers, the necking inwards of the edges of the dispensed material is more likely to be a problem.
Various expedients are known by those skilled in the art for controlling this tendency of the dispensed material to pull inwards. The art is replete with mechanical aids to draw the dispensed material back to a predictable width. These are often called “edge guides” in the literature. They are particularly to be seen in descriptions of slide and curtain coating.
However, literature is silent about what might be considered the opposite problem. Recently, attempts to coat high value materials (substrates) in very thin dry layers at very low speeds have resulted in coating of erratic width as capillary forces draw the dispensed material laterally along the gap between the die surface and the substrate at the ends of the applicator slot. This is because the thin dry layers coated onto the high value materials are diluted in a solvent for delivery to the substrate, which reduces viscosity and increases the coating thickness of the coating and solvent mixture delivered to the substrate.
Improvements are desired.